This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The Ca2+ release channel (also known as the ryanodine receptor, RyR) belongs to a family of integral membrane Ca2+ channels and is the largest known ion channel. In mammals, there are three homologous RyR isoforms. Their 70% sequence homology accounts for the many functional similarities between them. RyR1 is found primarily in skeletal muscle and mediates the release of Ca2+ from sarcoplasmic reticulum (SR) stores into the cytoplasm during contraction. RyR1 is the most studied due to its high concentration in the SR membrane and its relatively simple purification. The channel is composed of four subunits, each ~565 kDa, constituting a single homotetrameric cation-selective channel pore in the SR membrane. Mutations in this channel are associated with several human muscular genetic diseases such as malignant hyperthermia (MH) and the myopathy called central core disease (CCD). We are using single particle cryoEM to determine its highest possible resolution structure at different physiological states and cryoEM restrained modeling to determine folds of different domains.